Displaying a television image on a graphics screen generally requires a television signal defining the image to be digitally sampled and stored in a digital memory referred to as a frame buffer. Sampling and storage permits a time base of the television-image signal to be corrected - including, if necessary, a time compression of the television-image in order to show both television images and graphics images on the same screen. If it is desired to position a television image in an arbitrary window of the graphics screen, it will ordinarily be necessary to scale the dimensions of the television image down or up in the process of sampling. For some applications, it may be desirable that the image itself not be a full-screen image, but to be an arbitrary window located within the full-screen image.
FIG. 1 shows the mapping of a television source window 2 designated Ws into a high-resolution display-screen destination window 4 designated Wd. The following additional designations are used in FIG. 1: Ls--the number of lines in the full-screen television image; Ps--the number of samples representing an active part of the full-length television line; Xs,Ys--the width and height, respectively, of the television window measured in number of pixels and lines; xs and ys--coordinates of a pixel in the television window, relative to the upper left corner of this window; ps, ls--coordinates of the upper left corner of the television source window relative to the full television screen. Correspondingly, Ld designates the number of lines of the high-resolution destination screen. Pd represents the number of pixels read from the frame buffer and video refreshed during an active line of the high-resolution raster; Xd and Yd represent the length and height, respectively, of the destination window in the high resolution display where the television source window is to be mapped; xd, yd--coordinates of a pixel inside the destination window; pd, ld--coordinates of the upper left corner of the destination window relative to the upper left corner of the high-resolution display screen. For displaying a television image on a graphics screen, several features are desirable:
The full-size television screen image should correspond to the high-resolution display screen in terms of number of pixels per line and number of lines.
The sampling process should preserve a correct format ratio of image height to width. In other words, an object shape should not be distorted in the sampling process. For example, a circle should not become an oval.
It is advantageous to be able to select the ratio of height of the television source window to the height of the graphics-display destination window to be any desired rational number. Similarly, it is advantageous to be able to select the ratio of the width of the television source window to the width of the graphics-display destination window to be any desired rational number. Moreover, the selection of the two transformation ratios should be mutually independent. This provides for the mapping of an arbitrarily-chosen rectangular television window into any arbitrary rectangular graphics-display window.
The existing approaches attempting to satisfy the requirements set forth above involve an analog-scaling technique or a pixel-interpolation technique, both of which typically require bulky and expensive hardware.
The analog-scaling technique generally requires that sampling frequencies be manipulated or that the frequency of a video refresh clock be changed Neither of these two frequency-adjustment methods works well in vertical direction. Moreover, they typically provide only a restricted set of transformation ratios.
The pixel-interpolation hardware must be fast enough to satisfy live video-image sampling requirements. Moveover, the interpolation must be done on pixels of three colors (red, green and blue) in parallel. Consequently, interpolation hardware for color television is tripled in comparison with interpolation hardware for monochrome television images. In addition, the standard television coding schemes,--NTSC (USA, Japan),
(Germany, England), or SECAM (France, U.S.S.R)--are all based not on red/green/blue ("RGB") color video representation, but on luminance/chrominance ("Y/C") color video representation. Luminance/chrominance representation allows the video composite signal to take about half the bandwidth--each separately--than the bandwidth required for transmission of three signals encoding one of the three primary colors. Similarly, storage of digitized luminance/chrominance signals requires about half the memory than is required for storing digitized signals representing three primary colors.
For the reasons noted above, conventional analog-scaling methods and pixel-interpolation methods are principally limited to still-image capture systems, or to display of monochrome television images, or to television images of substantially reduced size and with substantially reduced frame-refresh frequency.
In general conventional analog scaling and interpolation techniques are too expensive to be used for displaying live television images on the graphics displays of low and medium cost workstations.
A recently-introduced digital-television technique is based on the luminance/chrominance representation of color television images for decoding, processing and storage. Depending on the television standard, the digital-television technique uses only one or two fixed standard sampling frequencies to provide digital processing of a television image, including digital decoding and brightness and hue control. The sampling frequency is chosen in view of certain characteristics of the television signal. In particular, the sampling frequency is fixed and is defined by reference to a multiple of the color subcarrier frequency in order to simplify the decoding and control of the color television signal. For example, one digital-television technique, referred to as the "ITT Intermetall" system uses a sampling frequency of 14.32 MHz which is four times the NTSC subcarrier frequency of 3.58 Mhz. In the ITT Intermetall system, each active television line is represented by 760 samples of luminance information and 380 samples of chrominance information. Each sample of luminanace is represented by 8 bits and each sample of chrominance is represented by 8 bits. The data structure is illustrated in FIG. 2. Chrominance C consists of two components, respectively designated "R-Y" and "B-Y." With digital data representing each of the three components luminance Y, R-Y chrominance and B-Y chrominance available, data representing the red R, green G and blue B primary colors may be derived according to certain rules. The ITT Intermetall procedure also includes a time-multiplexing technique to represent the luminance and chrominance information as 12-bit samples.
A second digital television technique is designated "CCIR 601" or the "4:2:2 standard." The digital-television system uses a sampling frequency of 13.5 Mhz which is approximately a multiple of horizontal frequency used in the SECAM or PAL color television system, namely 625 lines per frame, 50 frames per second, and a multiple of the horizontal frequency used in the NTSC color television system, namely 525 lines per frame, 60 frames per second. When using the 4:2:2 standard for sampling of NTSC television images, every active line is represented by 720 samples for luminance and 720 samples for chrominance.
The Philips Company markets integrated circuits for digital television processing which are based on the 13.5-Mhz sampling frequency, but which provide reduced chrominance resolution, as in case of the ITT Intermetall system. A time-multiplexing technique is also used to represent the sampled luminance/chrominance data by 12 bits per sample. For each television line, the Philips circuits provide 720 samples for luminance and 360 samples for chrominance.
There are several problems with adapting one of the conventional digital-television techniques to display television images on high-resolution graphics displays. Two problems arise from the use of a fixed sampling frequency and time multiplexing of luminance/chrominance data.
Use of a fixed sampling frequency precludes the use of the conventional analog-scaling approach to image transformation described above which involves changing the frequency of the sampling clock. Moreover, the time-multiplexed luminance/chrominance data format does not permit the use of the conventional pixel-interpolation technique for color pixels, without first taking additional time-consuming steps to convert the luminance/chrominance data to pixels representing the primary colors. After processing by the pixel-interpolation technique, the resulting pixels, which still represent the primary colors, must be converted back to the original luminance/chrominance format if the data is to be stored most compactly in digital memory.
A serious problem with adapting a conventional digital-television technique for displaying a television image on a high-resolution graphics display is that virtually no conventional graphics display uses the combination of number of pixels per line and number of lines per screen employed in the conventional digital-television schemes. Moreover, digital luminance/chrominance samples from a television line typically have an effective height-to-width ratio of substantially less than one. In contrast, conventional graphics displays, ordinarily have pixels with an effective height-to-width ratio of approximately one; that is, conventional graphics displays have essentially "square" pixels. The difference in effective height-to-width ratios is a consequence of differences in the requirements for the video clock in conventional graphics displays and for the sampling clock in conventional digital-television systems.
More specifically, the frequency of the video clock for a graphics display is generally based on requirements for the resolution of the display screen, relative to a size of a rectangular matrix of pixels displayed on the graphics display. Also, as noted above, the pixel height-to-width ratio is generally equal to essentially one in a graphics display. Use of such a "square" pixel makes it easier to calculate the coordinates of the pixels for representing vectors or polygons to be displayed on the display screen.
The width-to-height ratio of an active display area of a screen--be it a display screen of a graphics display or a television screen--is referred to as the "screen format ratio." Thus, for example, in a conventional graphics display with an effective pixel height-to-width ratio of essentially one and a screen format ratio of four to three, the number of pixels in an active line is equal to 4/3 of a number of active lines. The Video Graphics Adaptor for IBM "PS/2" workstation family has a resolution of 640 by 480 pixels, which corresponds to a screen format ratio of 4/3. Another IBM graphics adaptor designated model No. 8514A has a resolution of 1024 by 768 pixels, which also corresponds to a 4/3 screen format ratio. The present-day standard television receiver also has a screen format ratio 4 to 3, however, one proposed standard for high-definition television ("HDTV") has a screen format ratio of 16 to 9.
The problems of attempting to adapt conventional digital television techniques to show live television images on a conventional graphics display become even more complicated for window applications.